1. Field of the Invention
The present invention relates to an optical system for signal light detection in a flow particle analysis apparatus used for cell analysis or the like, and particularly relates to an optical system for signal light detection in which little variation occurs in the output signal of a photodetector even in the case where the position of the flow particles fluctuates.
2. Discussion of the Prior Art
A flow particle analysis apparatus is known which has a flow cell, a light source for illuminating flow particles within the cell, and an optical system for detecting signal light emitted from the flow particles. The flow cell is formed of an optically transparent material (for example, quartz), and a floating liquid of particles to be measured is caused to flow into a measuring flow path formed through the flow cell under the condition that the floating liquid is enclosed by a sheath liquid. The particles to be measured are caused to flow along a central portion of the measuring flow path so as to form one line by hydrodynamic focusing.
The particles to be measured flowing into the flow cell are irradiated with light such as laser light or the like from the light source. At this time, the scattered light generated from the particles to be measured, or fluorescence generated from the particles to be measured in the case where the particles to be measured are dyed with fluorescent pigment, is converged by the signal light detection optical system and converted into an electric signal. The magnitude of the electric signal corresponds to the intensity of the scattered light or fluorescence, and therefore analysis of particles, for example, of a blood cell or the like, can be performed on the basis of the electric signal.
FIG. 2 shows a signal light detection optical system 11 in a conventional flow particle analysis apparatus. In FIG. 2, flow particles p.sub.1 are flowing in the direction perpendicular to the paper plane. The flow particles p.sub.1 enter a flow cell 20 and are irradiated with laser light l from laser source 18 through a condenser lens 12.
Signal light (scattered light or fluorescent light) from the flow particles p.sub.1 is converged by a condenser lens 13 and focused on a plane of an aperture 14. The signal light which passes through the aperture 14 strikes a photosensitive surface 16a of a photodetector, for example, a side-on type photomultiplier tube 16 or the like.
In the photodetector, generally, if the incident position of the light varies, the detection sensitivity varies. Accordingly, if the flow particles move away from the central position to a position p.sub.2 as shown in FIG. 2, the signal light is displaced as shown by broken lines in FIG. 2 so as to change the incident position 16a of the light onto the photomultiplier tube 16. As a result, a disadvantage occurs in that the magnitude of the output signal of the photomultiplier tube 16 varies correspondingly.